1. Field of the Invention
The present invention relates to a method for measuring an absolute steering angle of a steering shaft for a vehicle, and, more specifically, to a method for measuring an absolute steering angle of a steering shaft by using two rotatable bodies that rotate together with the steering shaft at a predetermined rotation ratio.
2. Description of the Related Art
In general, measurement of an absolute steering angle of a steering shaft using an angle sensor only is known to be difficult because the measurement range is greater than 360°.
Also the steering angle of the steering shaft should be immediately measured following start-up of a vehicle, regardless of an initial angular position. However, a prior steering angle would not be used to measure a relative change at present stage.
U.S. Pat. Nos. 5,930,905 and 6,466,889B1 disclose a method for measuring an absolute steering angle of a steering shaft based on rotational angular measurements of a first rotatable body and a second rotatable body that rotate together with a steering shaft at a predetermined rotation ratio.
In the disclosures, the absolute rotation angle of the first rotatable body and of the second rotatable body are expressed by Ψ=Ψ′+iΩ and θ=θ′+jΩ, respectively (wherein, Ω indicates a measurement range of an angle sensor measuring the Ψ′ and the θ′; i is a whole number representing the number of times when the first rotatable body's absolute rotation angle Ψ is greater than the Ω, i.e. a frequency of the first rotatable body; and j is a frequency of the second rotatable body), and the absolute steering angle, Φ, can be obtained through a specific calculation procedure using measurements of Ψ′ and θ′.
According to the U.S. Pat. No. 5,930,905, the measurements of Ψ′ and θ′ are substituted to the following equation (1), which is derived from a geometrical relation among Ψ, θ, and Φ to get k, and by rounding off k, a whole number k is obtained. Then the k, Ψ′ and θ′ are substituted to the following equation (2) to obtain Φ.k={(m+1)Θ′−mΨ′}/Ω  <Equation 1>Φ={mΨ′+(m+1)Θ′−(2m+1)kΩ}/2n  <Equation 2>
(Here, m indicates the number of gear teeth of the first rotatable body; m+1 indicates the number of gear teeth of the second rotatable body; and n indicates the number of gear teeth formed on the steering shaft engaged with the first and second rotatable bodies.)
On the other hand, according to the U.S. Pat. No. 6,466,889B1, the steering angle, Φ, can be obtained directly from a relation between the difference of absolute rotation angles of two rotatable bodies, Ψ−θ, and ‘i’ of the first rotatable body (or ‘j’ the second rotatable body). Here, Ψ−θ is obtained by adding Ω to a measurement of Ψ′−θ′ if the measurement is a negative value, or by applying a measurement of Ψ′−θ′ if the measurement is not a negative value. The ‘i’ is calculated from the relation between Ψ−θ and i. Ψ is calculated from the known values of Ψ′ and i. Based on these values, the absolute steering angle of a steering shaft, Φ, is obtained.
When ‘i’ becomes k1 as the steering shaft is fully rotated, the rotation angle difference Ψ−θ should be equal or less than the measurement range of the angle sensor, namely Ω (cf. in the U.S. Pat. No. 6,466,889B1, Ψ−θ is set to be equal to Ω). In other words, the rotation angle difference Ψ−θ successively varies from 0° to Ω until the steering shaft is fully rotated, and i-value varies step by step from 0 to k1.
In particular, the U.S. Pat. No. 6,466,889B1 made an assumption that Ψ−θ and i−value are in a linearly proportional relation with each other, meaning that the value for i successively varies from 0 to k1 as the rotation angle difference Ψ−θ successively varies from 0° to Ω. Also, the value of ‘i’ is obtained by taking a maximum whole number that is smaller than a value obtained from the multiplication of Ψ−θ measured value and k1/Ω. For example, if ψ−θ times k1/Ω is 5.9, i is 5.
However, the above method poses a problem that ‘i−j’ has to be either 0 or 1 and should not be greater than 2 because a maximum value of Ψ−θ cannot be greater than Ω.